We will continue to define the DNA sequences which regulate transcription of the ribosomal genes of the frog, Xenopus laevis. Specific projects are: 1) Define the internal structure of the ribosomal gene promoter. 2) Study the mechanism whereby repeated elements in the spacer influence promoter activity. 3) Study specific regulatory events such as nucleolar dominance and the effect of amino acid starvation. 4) Study the mechanism of termination.